Boiling Water Reactors (BWR's) designed for power generation utilize fuel assemblies arranged inside vertical channels through which water coolant flows. Each of the fuel assemblies consists of a plurality of vertical rods arrayed within the vertical channels. The vertical rods are sealed cylindrical tubes which have ceramic pellets of fissionable material, (e.g. uranium oxide), stacked inside. The water flows upward in the channels and removes the heat generated in the pellets by the fission of the heavy isotopes. In addition to its cooling function, the water serves as a neutron moderator. The moderator function is achieved as the neutrons produced in the fission process collide with the hydrogen atoms in the water molecules and slow down to lower energies which increase the probability of inducing further fission reactions and the fission chain reaction is sustained.
In Boiling Water Reactors, the water is allowed to boil as it travels up in the fuel assembly channel. The density of water is reduced by the boiling process and the moderating function is reduced accordingly.
In the normal mode of operation of Boiling Water Reactors, the coolant flow rate through the fuel channels is steady and stable, however, departure from steady configuration is likely under reduced coolant flow operation, particularly when power levels are relatively high. Such operating conditions are encountered during reactor startup and as a result of recirculation pumps tripping—an anticipated transient. The mechanism of the instability is associated with the so-called density waves and is described as follows. Boiling Water Reactor fuel assemblies have a vertical boiling channel with initially steady inlet water flow rate. The density profile of the two phase mixture is one of monotonically decreasing density as function of elevation and is fixed in time. The density of the coolant at the exit of the channel is higher for higher coolant flow rate and is lower for higher power. Given a small perturbation in inlet flow rate, a corresponding perturbation in coolant flow density takes place at the boiling boundary and the density perturbation travels up the channel with the coolant flow, causing the density wave. The resistance to coolant flow increases substantially with decreasing flow density for the same mass flow rate. The density wave therefore affects the distribution of flow resistance along a boiling channel. In the specific case where the density wave travel time to the upper part of the channel coincides with the reversal of the inlet flow perturbation, a resonance effect results and the flow resistance change reinforces the original perturbation. The magnitude of the reinforcement is larger for high net density change, i.e. power-to-flow ratio, and can be sufficiently large to cause diverging flow oscillations, where the ratio of the magnitude of flow change at the peak of one cycle to that of the previous cycle (known as decay ratio) exceeds unity.
In a Boiling Water Reactor, the density waves cause corresponding changes in the moderating function of the coolant and periodically alter the reactivity of the core. The alternating reactivity results in corresponding neutron flux and power oscillations. These power oscillations filter through the fuel pellets, with damping and time delay caused by the heat diffusion process, and result in fuel surface heat flux oscillations. The heat flux oscillations interact with the density wave and generally reinforce it. It is noted that fuel rods of smaller diameter reduce the filtering effect and have an adverse effect on stability.
Early Boiling Water Reactor fuel designs utilized a simple array of 7×7 rods in a regular square lattice. The power density was relatively low, as the linear heat generation rate was relatively high, which forced the reactor power level to remain low to avoid set thermal limits. Newer designs use larger numbers of rods, specifically 8×8, 9×9, and 10×10 rod arrays. The increased number of rods resulted in decreasing the linear heat generation rate and permitted the fuel channel power density to increase, however, the increased number of rods resulted in two adverse effects:
The first adverse effect of increasing the number of rods is that the diameter of each rod is reduced. This results in proportional reduction in heat conduction time constant and reduces its stabilization effect.
The second adverse effect of the increase of the number of fuel rods in later designs is the increase of the coolant pressure drop as the hydraulic diameter of the subchannels is reduced. The two-phase flow resistance in the upper part of the flow channel is increased, which results in reduced hydraulic stability.
The development of large magnitude flow oscillations due to unstable density waves cannot be tolerated in a Boiling Water Reactor as it results initially in cyclical dryout and rewetting of the fuel surface and may lead to irreversible dryout. The occurrence of irreversible dryout leads to clad temperature increase and clad failure and release of radioactive material from therein. For this reason, Boiling Water Reactor plants take measures to guard against instabilities. These measures are:                1. Define by using computer simulations the boundaries of one or more exclusion zones on the power-flow map, where neutron-coupled density wave instabilities of the global or regional types are possible, and restrict operation in said zones.        2. Install hardware that accesses the neutron flux signals, and use these signals to determine if oscillatory behavior is present, in which case protective measures such as reactor scram are taken.        
There is therefore a need to provide a design that prevents density waves in Boiling Water Reactors while not exclusively dependent on their coupling to neutron flux signals.